Notes d'application

Porosity testing of enamel coatings on equipment used in the chemical and pharmaceutical industries

Because it is impervious to high temperatures and chemically reactive substances, vitreous enamel makes an excellent anti-corrosion barrier for the boilers and tanks used in the chemical and pharmaceutical industries. But this protection is only guaranteed if the coating is 100% continuous and has no pores, cracks, or other defects that could allow exchange between the equipment and its contents. This requires a reliable porosity test.

Steel tanks, pipes and mixers are often used in the manufacture of chemical and pharmaceutical products. To protect these parts from corrosion, they are coated with enamel 0.8-2.4 mm thick, depending on the requirements at hand. To ensure 100% protection, the enamel must be checked for gaps: To this end, the ISO 2746 standard test for enamelled articles for service under highly corrosive conditions specifies that the electrode of a high-voltage probe be moved slowly across the surface of the enamel. If the layer’s electrically insulating effect is interrupted (indicating a pore), the voltage drops suddenly. It is this voltage drop that is detected and signalled by the testing instrument.

Fig.1: Porosity test using the POROSCOPE® HV20 on a container in a chemical plant

The new POROSCOPE® HV20 is perfectly suited for the porosity testing of enamel coatings. The robust design of the probe head ensures longevity and meets all safety requirements for working with high voltage. Depending on the topology of the object to be tested, various electrodes are available for use, including flat brushes, sweepers or smaller whisks for testing inside of cavities.

Fig.2: Different electrodes are available for porosity testing

Porosity testing ideally takes place right after the enamelling process. The voltage applied is typically between 12 and 20 kV, depending on the thickness of the enamel layer. Based on the values laid out in the ISO 2746 standard, the correct voltage can be set directly on the probe head. Regular porosity evaluation during the fabrication of such pipes, tanks and mixers is the only way to verify the integrity of the enamel – a critical barrier that not only provides for corrosion protection of these components but also safeguards against contamination of their valuable contents.

The new POROSCOPE® HV20 is ideal for high voltage porosity testing of the vitreous and porcelain enamel coatings often used on equipment and infrastructure for the chemical and pharmaceutical industries. The adjustable range of 4-20 kV ensures that all common thicknesses can be reliably checked for defects such as cracks, pores and other weaknesses. For more information or a demonstration of the POROSCOPE® please contact your local FISCHER representative.

Using nanoindentation to characterise the hardness of coatings on weight-bearing medical implants

Sometimes, due to injury or disease, load bearing joints such as knees or hips must be replaced with artificial orthopaedics. Such surgeries are not only extremely painful but also expensive and risky; therefore, the longevity of these appliances is paramount: implants must be durable and generate absolutely minimal debris. Long service life is achieved through the use of tough coatings such as hard chrome or ceramics.

The replacement of a knee or hip is a major surgical procedure for renewing the weight-bearing surfaces, in order to relieve pain and restore functionality to the joint.

In order to fulfil their intended purpose, the replacement parts must meet very demanding requirements for wear resistance. Typically, there is a correlation between a material’s ability to withstand wear and its hardness; the Vickers hardness (HV) of a material is a metric that characterises this well. But traditional Vickers testers usually produce (only) a composite hardness of both the coating and the substrate, because the test loads used by these devices are too high. The accuracy of the results can suffer further from operator influence, as the diagonals of the indentation must be carefully measured – optically – under a microscope.

Nanoindentation is a well-established technique that uses very low loads to measure the HV of coatings when it is critical to avoid all influence from the substrate. The FISCHERSCOPE® HM2000 is the ideal instrument for this task, as it automatically calculates the HV from online monitoring of the penetration depth, thus eliminating any user error. Moreover, the elastic modulus can also be calculated using the slope of the unloading curve.

The study below uses the FISCHERSCOPE® HM2000 to measure hardness on an implant with a 7µm thick hard chrome coating. Applying a maximum load of 50mN produces an indentation depth well within the recommended 10% of coating thickness, as shown in Figure 1.

Fig.1: Load-Depth curve for six measurements on a knee implant coating

The HM2000’s integrated microscope and motorised XYZ stage allow for measurements to be taken directly on complex geometries like artificial knee implants – with minimal sample preparation. The average hardness of the chrome coating measured in this example was 1084.8 HV, with average elastic modulus of 279.7 GPa.

No patient or doctor wants a major joint replacement to wear out before its time. In order for surgically-implanted medical appliances to last, they must be coated properly. Nanoindentation is the method of choice for verifying this quality. The features and capabilities of the FISCHERSCOPE® HM2000 allow for accurate measurements to be made directly on the implants used in orthopaedic reconstruction. For more information, please contact your local FISCHER representative.

Hardness measurement of nano coatings on spectacle lenses

Whether used for eye protection or vision correction, spectacles with lenses made of plastic are preferred over glass for their considerably lower weight and better fracture strength. In order to provide the required life-long quality of such lenses a specific scratch-resistance is necessary.

Nowadays, spectacle lenses made of plastic are commonly provided with an anti-scratch, dirt-repellent and anti-reflective surface. They are vacuum coated using a physical vapour deposition (PVD) method with up to 10 protective layers, each only a few nanometres thick, which together ensure very high scratch-resistance. Hardness and scratch-resistance of these coatings are directly related: therefore, determining the hardness is a suitable method for quantifying the quality of these protective coatings.

To avoid commingling the hardness results of the coatings with those of the base materials while measur­ing, the test load must be absolutely minimal, as low as a few micronewtons: The indenter may only penetrate up to one tenth of the overall coating depth in order to correctly determine its hardness without being influenced by the properties of the substrate (Bückle’s-Rule).

Fig. 1: Hardness measurement of protective coatings of lenses (Martens hardness). Sample P4 has a significantly lower hardness and was identified as far less scratch-resistant (samples courtesy of Rodenstock)

Another important measuring parameter is the elastic/plastic deformation ratio of the coating material. The coatings must have a very high elastic component to prevent separation from the base material upon deformation. Therefore, multilayer coating systems are used that gradually adjust the modulus of elasticity from the base material to the top coating. These systems also have much higher adhesive bond strengths compared to single-layer coatings.

To secure the functionality of these protective coatings it is important to find the right balance between hardness and elastic behaviour.

The PICODENTOR® HM500 is ideal for measuring the hardness and elastic properties of these complex, nano-thin multi-coatings, which requires a measuring system capable of load generation as low as a few micronewtons and highly accurate depth measurement in the picometre range – exactly the designed operating range of the PICODENTOR®. The hardness can then be calculated from the measured load/depth curves. For further info contact your local FISCHER partner.